The present invention relates to a tracking control technology used in, for example, optical disk apparatus.
Optical disk apparatus performs tracking control for allowing a light beam to follow a track on an optical disk in accordance with a tracking error signal. A differential phase detection method is a method for the tracking control. If the center of a photodetector for converting light reflected from the optical disk into an electric signal is not on the optical axis of a lens for focusing the light beam (i.e., in the case of a lens shift), the DC level (direct current component) of a differential phase tracking error signal might deviate from a reference value or the amplitude of the differential phase tracking error signal might be small.
A tracking control system for generating a differential phase tracking error signal without being affected by such a lens shift is disclosed in Japanese Laid-Open Publication No. 11-175989. Hereinafter, an example of the known tracking control system will be described.
FIG. 17 is a block diagram showing an example of a configuration of the known tracking control system. In FIG. 17, each light receiving sections 11 and 12 provided on an optical pickup 10 converts a light spot SP, which is light reflected from an optical disk, into an electric signal and outputs the signal. V/I converting sections 21 and 22 convert the respective voltage signals output from the light receiving sections 11 and 12 into current signals, and output the signals to a differential phase tracking error signal generating section (differential phase TE generating section) 31. The differential phase TE generating section 31 obtains and outputs a phase difference between signals input thereto. An amplitude gain setting section 932 multiplies the output of the differential phase TE generating signal 31 by a constant which can be set at an arbitrary value.
A CPU 948 receives an output of a balance changing section 933 by way of a balance correcting section 934 and outputs a balance value for equalizing the upper part of the amplitude of the signal with the lower part thereof with respect to a reference value. The balance changing section 933 shifts the output of the amplitude gain setting section 932 in accordance with the balance value, i.e., changes the balance, thereby producing an output.
V/I converting sections 23 and 24 convert respective voltage signals output from the light receiving sections 11 and 12 into current signals and output the current signals to a differential amplitude tracking error signal generating section (differential amplitude TE generating section) 41. The differential amplitude TE generating section 41 performs a subtraction on the input signals and outputs the result. An amplitude gain setting section 942 multiplies the output of the differential amplitude TE generating section 41 by a constant which can be set at an arbitrary value and outputs the result.
An adding section 939 adds the output of the balance changing section 933 to the output of the amplitude gain setting section 942 and outputs the sum. An A/D converting section 34 converts the output of the adding section 939 into a digital value and outputs the digital value to a tracking control section 58. The tracking control section 58 generates a tracking control signal for performing tracking control of the optical pickup 10 in accordance with the output of the A/D converting section 34, and outputs the signal to a tracking drive section 59. The tracking drive section 59 performs tracking control, i.e., controls the position of the optical pickup 10 in accordance with the tracking control signal such that a light beam emitted from the optical pickup 10 follows a track on the optical disk.
An A/D converting section 44 converts the output of the amplitude gain setting section 942 into a digital value and outputs the digital value as a differential amplitude tracking error signal DTE. The differential amplitude tracking error signal DTE is used for control other than the tracking control.
The CPU 948 performs focus control for keeping the distance between the optical pickup 10 and the optical disk 2 substantially constant, using a focus drive offset section 71, a focus control section 72 and a focus driving section 73. If a defect on the optical disk is detected based on a signal output from a reproduced signal detecting section 15 on the optical pickup 10, the tracking control section 58 maintains the tracking control.
FIG. 18A is a cross-sectional view showing an orientation of the lens 14 and the light receiving sections 11 and 12, in an ideal case. FIG. 18B is a graph showing a waveform of a differential phase tracking error signal PT0 in the case shown in FIG. 18A. FIG. 18C is a cross-sectional view showing an orientation of the lens 14 and the light receiving sections 11 and 12 in the case where the position of the light receiving sections 11 and 12 shifts with respect to the lens 14. FIG. 18D is a graph showing a waveform of a differential phase tracking error signal PT1 in the case shown in FIG. 18C.
The optical disk 2 reflects a light beam applied thereto. Each of the light receiving sections 11 and 12 receives the reflected light and converts the light into an electric signal. In this case, it is assumed that focus control for keeping the distance between the optical disk 2 and the lens 14 constant is performed.
In the case shown in FIG. 18A, the boundary between the light receiving sections 11 and 12 is located on the optical axis CTL of the lens 14 for focusing the light beam. In this case, as shown in FIG. 18B, the differential phase tracking error signal PT0 is a signal with an amplitude AP whose center is at a reference potential DC0. An upper-part amplitude UAP which is a difference between the reference potential DC0 and the upper limit of the amplitude AP is substantially equal to a lower-part amplitude LAP which is a difference between the reference potential DC0 and the lower limit of the amplitude AP. Therefore, the differential phase tracking error signal PT0 is an ideal signal.
In the case shown in FIG. 18C, the light receiving sections 11 and 12 shift in the radial direction of the optical disk 2, and the boundary between the light receiving sections 11 and 12 is not on the optical axis CTL of the lens 14 for focusing the light beam. That is to say, a lens shift is present. In this case, as shown in FIG. 18D, a differential phase tracking error signal PT1 is smaller than in the case where the amplitude AP is in the state shown in FIG. 18B and the center of the amplitude AP is at a potential DC1 which deviates from the reference potential DC0. In addition, the difference between the upper-part amplitude UAP and the lower-part amplitude LAP with respect to the reference potential DC0 is enlarged. Therefore, the differential phase tracking error signal PT1 is a signal greatly lacking in balance.
The change in balance of a differential phase tracking error signal in the known tracking control system will be described. It is herein assumed that focus control is performed in this case. FIG. 18D shows a differential phase tracking error signal PT1 before a balance correction.
FIG. 19A is a graph showing a DC level DDC of a differential amplitude tracking error signal DTE in the case shown in FIG. 18C. The DC level DDC of the differential amplitude tracking error signal DTE which is a difference between a signal obtained from the light receiving section 11 and a signal obtained from the light receiving section 12 shifts from the reference potential DC0 in accordance with a distance for which the light receiving sections 11 and 12 deviate from the optical axis CTL of the lens 14 in the radial direction of the optical disk 2.
FIG. 19B is a graph showing a differential phase tracking error signal PT2 in the case of a balance change. If the differential phase tracking error signal PT1 is shifted by adding the difference between the DC level DDC of the differential amplitude tracking error signal DTE and the reference potential DC0 to the differential phase tracking error signal PT1, the upper-part amplitude UAP and the lower-part amplitude LAP can be made equal to each other with respect to the reference potential DC0.
FIG. 19C is a graph showing a differential phase tracking error signal PT3 in the case of correcting the balance and the amplitude. In the case shown in FIG. 18C, since the amplitude AP of the differential phase tracking error signal PT1 is smaller than in the case shown in FIG. 18A, the balance is corrected as shown in FIG. 19B, and then the amplitude gain is changed so as to increase the amplitude AP. Then, an ideal differential phase tracking error signal PT3 is obtained as shown in FIG. 19C.
Next, the case where the lens 14 for focusing a light beam has an aberration will be described. In this case, it is also assumed that focus control is performed. FIG. 20A is an illustration showing a positional relationship between the light spot SP and the light receiving sections 11 and 12 in the case of the absence of both an aberration of the lens 14 and a lens shift. If there is no lens aberration, the light spot SP is in the shape of an approximately perfect circle. If the light spot SP is on the boundary between the light receiving sections 11 and 12, the differential phase tracking error signal is an ideal signal having an upper-part amplitude UAP and a lower-part amplitude LAP which are equal to each other with respect to the reference potential DC0 as in the case shown in FIG. 18B.
FIG. 20B is an illustration showing a positional relationship between the light spot SP and the light receiving sections 11 and 12 in the case where the lens 14 has an aberration and exhibits no lens shift. In the case where the lens 14 for focusing a light beam has an aberration, a light spot SPL is in the shape of an ellipse which is inclined about 45 degrees, for example, toward the boundary between the light receiving sections 11 and 12. In this case, as in the case shown in FIG. 18D, the DC level of the differential phase tracking error signal deviates from the reference potential DC0 and there is a difference between the upper-part amplitude UAP and the lower-part amplitude LAP with respect to the reference potential DC0, and the amplitude AP is small.
FIG. 20C is an illustration showing a positional relationship between the light spot SPL and the light receiving sections 11 and 12 in the case where the lens 14 has an aberration and exhibits a lens shift. FIG. 20D is a graph showing a differential phase tracking error signal PT5 in the case shown in FIG. 20C. The differential phase tracking error signal PT5 in the case of the presence of both of the lens aberration and the lens shift has a DC level DC5 greatly deviating from the reference potential DC0, so that the difference between the upper-part amplitude UAP and the lower-part amplitude LAP with respect to the reference potential DC0 is large and the amplitude AP is small.
FIG. 20E is a graph showing a signal (PT6) when the differential phase tracking error signal PT5 in the case shown in FIG. 20C is shifted in the same manner as in FIG. 19B. In the case of a lens aberration, since the DC level DC5 of the differential phase tracking error signal PT5 greatly deviates from the reference potential DC0, a DC level DC6 of the differential phase tracking error signal PT6 does not coincide with the reference potential DC0 even after the shift.
FIG. 20F is a graph showing a signal (PT7) when the amplitude of the differential phase tracking error signal PT5 in the case shown in FIG. 20C is corrected in the same manner as in FIG. 19C. As shown in the drawing, even the amplitude is further changed, a DC level DC7 of the differential phase tracking error signal PT7 does not coincide with the reference potential DC0.
As described above, in the known tracking control system, if the aberrations of lenses for focusing a light beam vary among the lenses, a potential deviation of the DC level occurring in a differential phase tracking error signal cannot be corrected sufficiently.
In addition, a circuit for adding a differential phase tracking error signal to a differential amplitude tracking error signal is needed, and a circuit for multiplying a gain is also needed so as to change the amplitude of the differential amplitude tracking error signal arbitrarily. As a result, the cost is high.
Furthermore, when a light beam passes on a defect present on an optical disk or when tracking control is performed again after seek operation for transferring a light beam to a track on the optical disk, for example, an unnecessary signal component is mixed into a differential amplitude tracking signal to have a harmful influence on a differential phase tracking error signal. As a result, the tracking control is unstable.